Fast pyrolysis processor which produces low oxygen content, liquid bio-oil

ABSTRACT

In this fast pyrolysis processor the reaction conditions are tailored to minimize the production of gas, while using calcined limestone to provide the heat for fast pyrolysis of biomass and to lower the acidity and oxygen content of the liquid bio-oil which is produced.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable

FIELD OF THE INVENTION

The present invention relates generally to using biomass as a source ofrenewable energy. In particular the invention relates to the productionof low oxygen content pyrolysis oil from biomass.

BACKGROUND OF THE INVENTION

Biomass as an energy source is attractive for two main reasons. First,it is renewable. Second, the carbon which makes up the backbone of it'sstructure is drawn from the air, lowering the carbon dioxide levelswhich are currently causing great concern for the environment.

Unfortunately, the energy density of biomass is low. It is a bulkycommodity which can be expensive to handle and transport. Often the costof transportation is more than the value of the energy in the biomass.That is why biomass as an energy source is not currently costcompetitive with other options available on the energy market.

Numerous methods, such as baling, pelleting, grinding, gasifying, andliquefying have been used over the years to pack the energy content ofbiomass into a more easily transportable form. Unfortunately, most ofthe previously tried methods have not gained significant acceptance.Liquefaction is the method which is currently receiving the mostattention.

The oldest, and most widely used method of liquefying biomass is theancient art of fermentation. In the 1800's fermentation was expandedbeyond just sugars to include fermenting cellulosic ethanol. Cellulosicethanol was produced in Europe and America during the early 1900's, butit proved to be unprofitable and was eventually abandoned. Today, recentadvances in fermentation have caused a renewed look at this ancient art.But, fermentation is a slow process and ethanol facilities must be largeto be economically viable. These large facilities consume more biomassthan can be produced in the close surrounding area. In some instancestheir feedstock must be transported in over long distances. That is whyone of the biggest expenses in cellulosic ethanol production has alwaysbeen the cost of transporting biomass to large fermentation facilities.

A more recently discovered liquefaction process is fast pyrolysis. Brown(2003:182-183) says, “Fast pyrolysis is the rapid thermal decompositionof organic compounds in the absence of oxygen to produce liquids, gases,and char. The distribution of products depends on the biomasscomposition and rate and duration of heating. Liquid yields as high as78% are possible for relatively short residence times (0.5-2 s),moderate temperatures (400-600 C.), and rapid quenching at the end ofthe process.”

Fast pyrolysis can liquefy a lot of biomass with a relatively smallprocessor. In fact, the invention we disclose in this patent was madesmall enough that a one ton per day prototype is currently in use as asmall scale, trailer mounted, transportable pyrolysis processor. Thisprocessor can be taken directly to the site where biomass is produced,eliminating the cost of transporting biomass altogether.

In the past, fast pyrolysis oil never gained acceptance as an energysource because it was of low quality, and had little value as a fuel.The molecules which made up fast pyrolysis oils contained large amountsof oxygen in their structure. This made them unstable and gave them arelatively low heating value compared to petroleum based oils. Theyneeded to be upgraded and hydrogenated to make high grade fuel.Unfortunately, fast pyrolysis oil was also very acidic, having highconcentrations of organic acids. This acidity meant that normal refineryequipment would be quickly damaged while trying to upgrade typical fastpyrolysis oil.

Using Calcined Limestone in Pyrolysis Reactions

For many decades people have made use of calcined limestone, or moreaccurately calcined calcium carbonate (CaCO₃), in various ways toachieve a desired result from their pyrolysis inventions. In all theseinventions, the reaction conditions were tailored to promote thoseaspects of the calcining/carbonation cycle which the inventor felt wouldbe useful.

Calcining is a reversible chemical reaction that occurs when CaCO₃ isheated above a temperature which is heavily dependent on the level ofcarbon dioxide (CO₂) in the atmosphere. When CaCO₃ calcines, it absorbsheat, gives off CO₂, and turns into calcium oxide (CaO). In normal airthis begins at about 550° C. In a CO₂ rich atmosphere, such as is foundin a calcining chamber, the temperature will be much higher. Forcalcining to happen at a useful rate inside a calcining chamber thetemperature must be greater than about 900° C.

When the temperature of CaO is then lowered, it absorbs CO₂, releasesits stored heat and turns back into CaCO₃.

This release of CO₂ at high temperatures and the re-absorption of CO₂ atlow temperatures makes CaCO₃ an ideal heat carrier for our pyrolysisreactor.

In the past, most inventions which made use of the calcining/carbonationcycle of CaCO₃, were aimed at gasifying carbonaceous solids orextracting CO₂ from a gaseous mixture. Many of these gasifiers also madeuse of the reaction C_((s))+H₂O_((g))→CO+H₂ to draw carbon out of thematerial they sought to gasify and add to the yield of hydrogen gasproduced.

In 1915, Georges Claude received U.S. Pat. No. 1,135,355 for a processto produce relatively pure hydrogen by extracting carbon monoxide fromWater Gas with calcium hydroxide, which is the hydrated form of calcinedlime. This invention used CO absorption, but made no use of theexothermic nature of the absorption.

In 1953, Everett Gorin received U.S. Pat. No. 2,705,672 for his processwhich used steam and calcium oxide to produce Water Gas fromcarbonaceous solids in a high temperature, high pressure reaction.

In 1982, Shang-I Cheng received U.S. Pat. No. 4,353,713 for hisinvention to gasify coal. His invention injects water into the reactionto consume carbon by way of the C_((s))+H₂O_((g))→CO+H₂ reaction, and itinjects CO₂ from the combustion phase to keep CO₂ levels high in thepyrolysis zone. His invention used the exothermic heat of the CO₂absorption process to drive the pyrolysis reaction.

In 2004, Klaus S. Lackner received U.S. Pat. No. 6,790,430 for hisinvention to produce hydrogen from carbonaceous material. This inventionis much more involved than Mr. Cheng's or Mr. Gorin's, but it stillmakes use of water injection to help consume carbon and increase thehydrogen yield.

A survey of the many patents that use the calcining/carbonation cycle ofCaCO₃, shows that water or steam is usually added to increase theproduction of hydrogen and consume carbon by the reactions:C_((s))+H₂O_((g))→CO+H₂followed byCO+H₂O_((g))→CO₂+H₂

Also, in most of those previous patents, CO₂ levels were kept high toget more process heat from the exothermal carbonation of CaO.

SUMMARY OF THE INVENTION

In our invention we wish to reduce the oxygen content of the pyrolysisliquids. We do not know of any acceptable means of directly drawing pureoxygen out of a high temperature reaction. However, CaO will absorb CO₂out of a high temperature reaction. So, to accomplish our goal ofdrawing oxygen out of the pyrolysis liquids, we must sacrifice a portionof the carbon in the biomass to be able to extract the unwanted oxygen.

Please note, that in our application, although one carbon atom issacrificed, two oxygen atoms are extracted for each CO₂ moleculeabsorbed by the CaO. The net effect is a reduction in the amount ofoxygen bound up in the molecular structure of the pyrolysis liquidproduced.

At the same time, we wish to minimize the production of non condensablegases and also minimize the loss of carbon by the reactionC_((s))+H₂O_((g))→CO+H₂. To do this we must keep the amount of H₂O inthe reaction chamber as low as possible.

To accomplish these goals, we use the calcining/carbonation cycle ofCaCO₃ and tailor the reaction conditions to achieve the results wedesire.

This leads to the FOUR MAIN FEATURES that, when combined in the sameprocess, distinguish this pyrolysis process from others.

(1) The amount of CO₂ in the reaction chamber is kept as low aspossible.

CaO absorbs CO₂ from the reaction zone, not directly from the moleculesof the vaporized pyrolysis liquids. According to Le Chatelier'sPrincipal, for this to effectively draw CO₂ out of the molecules of thevaporized pyrolysis liquids, the amount of CO₂ in the reaction zone mustbe kept as low as possible. This is in stark contrast to gasificationprocesses where CO₂ levels are kept high by introducing CO₂ producedelsewhere into the reaction zone, to get more process heat from theexothermic conversion of CaO to CaCO₃.

(2) The pyrolysis temperature is kept between about 700° C. and 450° C.

The product we seek from our invention is pyrolysis liquid, not gas. Tomaximize the production of liquid and minimize the production of gas thetemperature of the pyrolysis reaction must be kept below about 700° C.For the pyrolysis to occur at a fast enough rate to be useful, and notproduce excessive amounts of char, the temperature must be kept aboveabout 450° C.

(3) H₂O in the reaction chamber is kept as low as possible.

To minimize the undesirable conversion of the biomass to hydrogen gas,the amount of water in the reaction chamber is kept as low as possibleto reduce the consumption of carbon by the reactionC_((s))+H₂O_((g))→CO+H₂.

(4) The calcined CaCO₃ bearing granular material is cooled to less thanabout 700° C. before entering the reaction chamber.

To calcine CaCO₃ and produce CaO fast enough for the process to beuseful, it must be heated to over about 900° C. At that temperature CaOwill not absorb CO₂ from a pyrolysis reaction. Therefore, aftercalcining, the CaO must be cooled to less than about 700° C. beforebeing brought into contact with the biomass, in order to be able toextract CO₂ from the pyrolysis reaction.

The Calcining Oven

We designed our calcining oven with separate upper and lower chambers.Calcining is done in the upper chamber. Then, in the lower chamber,where there is no heating; the calcined lime is given time in a lowerCO₂ environment so that it can continue giving off CO₂ while itstemperature equilibrates. In the upper chamber, the limestone is quicklyheated until it's outer fringes are bright yellow (about 1200° C.). Theheating is done quickly so that the main body of the granule does notget as hot. The goal is to put just enough heat into the granule so thatwhen it passes into the lower chamber and has time to thermallystabilize, the final equilibrium temperature will be about where we wantto do our pyrolyzing. For maximum liquid yields this would be between700° C. and 450° C. The precise temperature of the granules leaving thelower chamber is ultimately controlled by varying the small amount offresh air that is allowed to flow through the lower chamber to keep CO₂levels low.

What comes out of the oven is CaCO₃ granules, at the desired pyrolyzingtemperature, with a thin coat of CaO on the outside that acts as astrong CO₂ absorbent and heat carrier.

Neutralizing Acids in the Pyrolysis Oil

Organic acids, such as the acetic acid which forms from acetyl radicalsin the pyrolysis process, are quite prevalent in pyrolysis oils, andcause problems when trying to use, refine, or upgrade the oil. Inaddition to the benefit of lowering the oxygen content of the productsof pyrolysis, it appears that including calcined CaCO₃ in the pyrolysisprocess can also lower the acidity of the pyrolysis products.

A test of the aqueous phase from one run of our processor showed a PH of7.6, as opposed to reports in the literature of other researcher'spyrolysis oils which typically have PH readings in the 2.5 range.

DESCRIPTION OF THE DRAWING

The drawing shows a side view diagram of the preferred embodiment of theinvention.

DESCRIPTION OF THE INVENTION

CaCO₃ bearing granular material (hereafter, simply referred to as the“heat carrier”) is calcined in a continuous flow oven (A). The calcinedheat carrier flows down into a cooling chamber (B), where it cools downto the pyrolysis temperature. A shuffle tray feeder (C) regulates theflow of the heat carrier into the mixer(G).

Finely chopped biomass is held in a bulk tank (D), behind the oven. Thebiomass is dried and conveyed into the pyrolysis chamber by an auger(E), which has an external heating jacket that is heated by exhaust fromthe calcining oven, carried through pipe(F).

The dried biomass is mixed with the calcined heat carrier in a rotatingdrum mixer(G), which serves as the pyrolysis chamber. Inclined bats(Gb),inside the rotating drum mixer, churn the mix and move it forward.

The pyrolysis gas is drawn out through a pipe (H) from the tail end ofthe pyrolysis chamber. It is cleaned of ash and char by a hot gascyclone (I) and then cooled and separated into a liquid portion and anon condensing gaseous portion in a unit (J) of our own design, whichwill be described in a later patent.

After the biomass is pyrolyzed in the mixing chamber, the heat carrierand remaining ash drop into a bucket elevator (K) and are carried up toa screening auger(L), mounted above and roughly parallel to the mixingchamber. The screening auger separates the ash from the larger sizedheat carrier and drops the ash into a hopper to be used later asfertilizer.

The heat carrier is then dropped into a holding tank (M), where excessheat carrier is held until needed to replace heat carrier lost toattrition during cycling through the processor. From the holding tankthe heat carrier flows down into the top of the oven (A), where it beganits cycle through the processor.

Testing of the Pyrolysis Products

Tests of the acidity of the two phase liquid product produced by thisapparatus were done by a national research laboratory. Their testsshowed a Total Acid Number (TAN) of:

Aqueous Phase—17.65

Organic phase—19.04

An elemental analysis showed the oxygen content of the organic phase ofour pyrolysis oil to be less than 25% on a wet, as produced basis. Mostother fast pyrolysis oils tend to have oxygen contents in the 40% to 50%range.

I claim:
 1. A method of producing low oxygen content pyrolysis liquid,while minimizing the production of gas, from the pyrolysis of biomasscomprising: (a) calcining CaCO₃ bearing granular material at atemperature greater than about 900° C. (b) cooling the calcined CaCO₃bearing granular material from greater than about 900° C. down to thedesired pyrolysis temperature (c) rapidly mixing the calcined CaCO₃bearing granular material at the desired pyrolysis temperature withbiomass feedstock so that: 1) heat from the calcined CaCO₃ bearinggranular material causes fast pyrolysis of the biomass feedstock 2) CaOin the calcined CaCO₃ bearing granular material absorbs CO₂ from thepyrolysis products (d) drawing the gaseous and suspended particulatepyrolysis products out of the pyrolysis chamber and separating them intoa particulate portion composed primarily of char and a clean gas portion(e) cooling the clean gas portion, thereby condensing a liquid fractionand leaving a non-condensing gas fraction (f) removing the CaCO₃ bearinggranular material and pyrolysis ash from the pyrolysis chamber,separating them, and returning the CaCO₃ bearing granular material tothe calcining step.
 2. A Method as claimed in claim 1, wherein the CaCO₃bearing granular material is partly or completely comprised of crushedstone from the group: Chalk, Limestone, Marble, or Travertine.
 3. Amethod as claimed in claim 1, wherein the CaCO₃ bearing granularmaterial is calcined at a temperature less than about 1600° C.
 4. Amethod as claimed in claim 1, wherein the desired pyrolysis temperatureis between about 700° C. and 450° C.
 5. A method as claimed in claim 1,wherein the separated particulate portion composed primarily of charfrom the gaseous and suspended particulate pyrolysis products is used asfuel for the calcining of the CaCO₃ bearing granular material.
 6. Amethod as claimed in claim 1, which produces pyrolysis liquids withreduced acidity.
 7. A method as claimed in claim 1, wherein the biomassfeedstock is dried so as to minimize the amount of H₂O taking part inthe pyrolysis reaction.